1. Field of the Invention
The present invention relates to a fluorescence observing apparatus for measuring fluorescence emitted from a sample (e.g., an organism, etc.) by irradiation of excitation light to provide information which is used for diagnosis, etc., and more particularly to a fluorescence observing apparatus that employs a semiconductor laser as its excitation light source.
2. Description of the Related Art
A diagnosis instrument, etc., for acquiring the intensity and spectrum of fluorescence emitted from a sample (e.g., an organism, etc.) by irradiation of excitation light to obtain information which is used for diagnosis, are known. These diagnosis instruments employ a method of detecting fluorescence emitted when excitation light for diagnosis is irradiated to the tissue of an organism, a method of detecting fluorescence emitted by irradiating excitation light to the tissue of an organism which has beforehand absorbed a drug for fluorescence diagnosis, or similar methods. The diagnosis instrument is incorporated into an endoscope, a colposcope, an operation microscope, etc., and is utilized for observation of a fluorescence image.
For example, Japanese Unexamined Patent Publication No. 9(1997)-327433 discloses a system that uses a mercury vapor lamp as an excitation light source in order to emit self-fluorescence from the respiratory organs and the stomach and intestines. In this system, a morbid tissue is detected by detecting self-fluorescence emitted from the tissue of an organism by irradiation of the excitation light emitted from the mercury vapor lamp. It is desirable that the excitation light for emitting fluorescence from the tissue of an organism have a wavelength belonging to a short wavelength region from ultraviolet rays to visible light. Mercury vapor lamps can easily obtain high output in this wavelength region.
Also, Japanese Unexamined Patent Publication No. 59(1984)-40830 discloses an apparatus which employs an excimer dye laser as an excitation light source. In this apparatus, the excitation light emitted from this light source is irradiated to the tissue of an organism into which a photosensitive material having tumor affinity has been injected beforehand, and the fluorescence emitted from the tissue is observed. The above-mentioned technique is used for observing the tissue of an organism as a dynamic image by obtaining an image from the tissue at cycles of 1/60 sec and is capable of simultaneously observing a normal image and a fluorescence image as the dynamic image. For observation of the fluorescence image, the excitation light emitted from the excimer dye laser is irradiated to the tissue of an organism (which is a subject) with a pulse width of 30 nsec at cycles of 1/60 sec, and the fluorescence emitted from the tissue by irradiation of the excitation light is imaged by a high-sensitivity imaging device for a fluorescence image. In this way, the dynamic image is obtained. On the other hand, for observation of the normal image, white light is irradiated to the tissue of an organism (which is a subject) at cycles of 1/60 sec, while the aforementioned period of the irradiation of the excimer dye laser which is performed at cycles of 1/60 sec with a pulse width of 30 nsec is being avoided. The obtained images are formed as the dynamic image by an imaging device for a normal image.
Here, the pulsed light emission of an excimer dye laser will be output as pulsed light whose peak value is extremely high, even if the emission time is 30 nsec. Therefore, the intensity of fluorescence being emitted from the tissue subjected to the irradiation is sufficient to obtain satisfactory diagnosis information. In addition, there is almost no time lag between the irradiation of excitation light to the tissue and the emission of fluorescence from the tissue and therefore the irradiation of excitation light and the emission of fluorescence are considered nearly the same. Thus, there is no possibility that the period during which the irradiation of excitation light and the formation of a fluorescence image are performed will overlap with the period during which the irradiation of white light and the formation of a normal image are performed. Furthermore, because the formation of a fluorescence image is performed within the blanking period after the formation of a normal image which is a short time, the rate at which external light and background light (such as indoor illumination) are formed as noise components, along with the fluorescence image is extremely low.
As described above, while excimer dye lasers and mercury vapor lamps have many advantages as an excitation light source, the apparatus is extremely large in scale and extremely high in cost. Because of this, employing a small and inexpensive semiconductor laser as an excitation light source has recently been discussed.
The semiconductor laser, however, is weak in light intensity when employed as an excitation light source that is desired to emit light which has a wavelength belonging to a short wavelength region from ultraviolet rays to visible light. In addition, if the semiconductor laser is oscillated to generate a peak value greater than or equal to the continuous maximum rated output value, a phenomenon called catastrophic optical damage (COD) will arise and the end face of the active layer of the semiconductor laser will be destroyed. In this phenomenon, non-radiative recombination occurs from a defect in the end face of the active layer of the semiconductor laser, and non-radiative recombination energy is converted to heat by the thermal vibration of the lattice. Because of this heat, the temperature of the end face rises and dislocation propagates, whereby the bandgap becomes narrower. If the bandgap becomes narrower, the end face further absorbs light and generates heat, resulting in a rise in the temperature of the end face. As a result, thermal run-away occurs and finally melts the end face. Particularly, in the semiconductor laser with a large energy gap, which is employed in an excitation light source to emit light which has a wavelength belonging to a near ultraviolet region, it is difficult to inject a large current to enhance light output and also difficult to stably emit pulsed light having a peak value greater than or equal to the continuous maximum rated output value.